Deformation Mechanisms in Nanocrystalline Copper Alloys at Ambient and Cryogenic Temperatures

When materials are subjected to deformation, the microstructure evolves. In this research, the microstructure response to deformation is considered in two extreme conditions: cryogenic temperatures, and nanocrystalline grain sizes. Prior reports have shown nanocrystalline materials with heightened grain growth at cryogenic temperatures. Simulations have shown faster twin boundary migration at lower temperatures in some cases as a result of coordinated dislocation motion, contributing to grain growth. This research explored the influence of solutes in this migration. Nanotwins were fabricated via sputter deposited copper with 2 at% and 8 at% aluminum. The aluminum content increases the hardness of the alloy contributing to shearing behavior at 8%. Both samples revealed grain bending when indented. This bending promoted detwinning by imposing a shear stress on the twin planes. An increase columnar bending with corresponding detwinning was observed as temperature decreased in the Cu-2Al sample whereas the Cu-8Al sample varied minimally with temperature. Simulated mobilities of an incoherent twin boundary matched well with these results. Grain growth was not observed highlighting the importance of grain aspect ratio. Follow-on work investigated mechanical properties of these alloys with temperature and twin configuration. These nano-twinned alloys were sputter deposited with twins parallel to the substrate and at an angle. Micro-pillars were deformed in-situ at ambient and sub-ambient temperatures. All samples got stronger as temperature decreased and with twins parallel to the substrate. Post-mortem characterization suggests that detwinning played a significant role as dislocations avoid cross-slipping through twin boundaries.Using nanocrystalline copper, deformation mechanisms were imaged with in-situ TEM testing. Digital Image Correlation (DIC) was applied to track the strain evolution. DIC has been minimally used in TEM imaging because of complex contrast mechanisms. This work demonstrates that DIC can produce reasonable strain values for TEM samples. This research captures strain evolution under deformation in nanocrystalline copper as a result of various dislocation mechanisms. By mapping the DIC strain onto the grain size, experimental verification of preferred plasticity within large grains is shown, supporting prior modeling predictions. Overall, this dissertation provides an understanding of the deformation of nanocrystalline copper and its alloys at ambient and sub-ambient temperatures.